1. Field of the Invention
The present invention relates to an improved spray nozzle for two-component, air-assisted, low pressure spray systems, which is especially suited for applying highly reactive, two-component coating or sealing compositions, preferably two-component polyurea coating compositions.
2. Description of the Prior Art
Several types of spray systems are known for applying two-component coating compositions. If the two components are not highly reactive, it is possible to mix the two components prior to use and apply the systems with known one-component, airless or air-assisted spray systems. These systems are generally used with coating compositions having a pot life of one hour to several hours.
Preferably, the two-component compositions are applied with two-component spray systems, which may be either high pressure (more than 100 bar) or low pressure (less than 100 bar) systems. The high pressure systems are usually airless or air-assisted airless spray systems. In these systems the two components are generally introduced under high pressure into a static mixer and are then passed through a spray tip under sufficient pressure to atomize the liquid. One disadvantage of these systems is their high cost.
Also suitable are low pressure or high pressure impingement mixers in which the components are introduced through separate orifices into a mixing chamber and then pass through an atomization spray tip under fluid pressure. The mixing chamber is generally purged with a purge rod or pressurized air. Disadvantages of impingement mixing systems are their cost, difficulty to use and typical high pressure requirements.
Examples of low pressure systems for applying two-component compositions are air-assisted sprayers in which the components are premixed and then siphoned or passed by gravity through an air atomization tip. Air is passed in a generally perpendicular manner to the mixed composition from opposing outlets to atomize the composition into the desired spray pattern. A disadvantage of these spray systems is that they are not suitable for spraying highly reactive two-component systems. Both during the spraying process and especially during stoppages, the components can react to form polymer solids that clog both the liquid spray tips and/or the air outlets. This causes a lengthy down time to clean the equipment for further use.
One alternative to prevent clogging in any of these low or high pressure sprayers is to flush the equipment with a solvent or an air purge prior to stoppages. However, this embodiment results in higher equipment costs due to the presence of a third stream. In addition, the use of a solvent is disadvantageous both from a cost standpoint and an environmental standpoint.
It is an object of the present invention to overcome the disadvantages of prior art spray systems in a cost effective manner.
This object may be achieved with the low pressure, two-component, air-assisted spray system according to the present invention. An important feature of the invention is the use of a disposable static mixer having an optionally removable spray tip to eliminate the clogging problems of prior art systems when spraying highly reactive two-component systems.
Copending application, U.S. Ser. No. 09/838,792, describes a static mixer having an optionally removable spray tip, but in that application the static mixer is not inserted into a shroud. U.S. Ser. Nos. 09/917,309 and 09/917,298 also describe static mixers having optionally removable spray tips, but these applications require the static mixers to be used in combination with caulking guns.
The present invention relates to a spray nozzle for a two-component, air-assisted, low pressure spray system including
a) a static mixer having an inlet port on its upstream end for receiving a first liquid component and a second liquid component that is reactive with the first liquid component and having on its downstream end an optionally removable spray tip for introducing the mixed liquid components into an atomizing zone,
b) a static mixer shroud, which encases the static mixer, is dimensioned on its upstream end for connecting the shroud to a dispensing unit for the first and second liquid components and has on its downstream end an optionally removable spray section having inlet ports for receiving pressurized air and outlet ports for introducing air into the atomizing zone, wherein the spray tip passes through an orifice in the spray section such that the mixed liquid components are introduced into the atomizing zone.
The present invention also relates to a process for spraying a two-component composition by
a) introducing a first liquid component and a second liquid component, which is reactive with the first liquid component, into a static mixer having an optionally removable spray tip,
b) mixing the first component with the second component in the static mixer,
c) discharging the mixed liquid components from the static mixer through the optionally removable spray tip at the downstream end of the static mixer into an atomizing zone,
d) introducing air into the atomizing zone,
e) atomizing the mixed liquid components in the atomizing zone,
f) spraying the atomized liquid components onto a substrate and
g) during an interruption in the spraying process or after termination of the spraying process, discarding the static mixer and the optionally removable spray tip.